DESCRIPTION: (Investigator's abstract) Primitive stem cells are found in many adult tissues and, throughout life, replenish mature cells lost through attrition or injury. Although thought to be committed to differentiate into cells of their native tissues, some stem cells now appear capable of generating several different cell types, an indication that they have retained considerable plasticity in their genetic programs. The biologic and therapeutic implications of "transdifferentiation" potential are far reaching but will remain speculative until the concept is better substantiated and the mechanisms that might drive this process are known. Evidence from the applicant's laboratory indicates that adult murine skeletal muscle contains stem cells capable of complete hematopoietic regeneration. Thus, the central hypothesis to be tested is that these stem cells are a subset of myogenic stem cells known to participate in muscle regeneration, they share certain characteristics of bone marrow stem cells, have myogenic activity, are transducible with retroviral vectors, and can be found in human muscle. The major objectives of this proposal are to characterize the biologic properties and markers of muscle-derived hematopoietic stem cells by cell sorting and bone marrow transplantation experiments (Aim 1) so that the cells can be readily identified and purified, and their salient features an be compared with those of marrow-derived stem cells. It will also be important to assess the in vitro and in vivo myogenic potential of purified and clonal populations of muscle-derived hematopoietic stem cells (Aim 2). The results will establish whether or not the stem cells are bipotential precursors of both muscle and blood and therefore an extremely primitive stem cell with a flexible program of differentiation. If the muscle stem cells can be readily transduced with retroviral vectors and the modified cells shown to engraft and stably express a marker gene in transplant recipients (Aim 3), they may afford a novel means of delivering therapeutic genes of interest. Finally, results obtained in mice will need to be verified, both in vivo and in vitro, with stem cells derived from human muscle (Aim 4). Answers from these studies will be crucial in determining if human muscle could provide a source of hematopoietic stem cells for therapeutic exploitation. Successful completion of these aims will not only provide an estimate of the hematopoietic potential of adult muscle-derived stem cells, but also a foundation for future studies into the mechanisms of stem cell transdifferentiation.